I've noticed my rear brakes (disc) dragging on my truck. The first time I noticed it was after driving about 15min, and finishing with an uphill, followed by a couple of moderate stops. When I got out the brakes smelled, and had developed enough heat that the rear rims were warm to the touch. The fronts were cold, though there was warmth coming off the front rotors. On recommendation from a mechanic friend (and a good one), I replaced the rubber line between the chassis and rear axle, since he said they can detereorate and get blocked up. I cut the old one up, and it looked fine, so I don't think this is the problem. The caliper slides are lubed, the pistons move properly, and the pads are not binding. I also visually checked all the lines to the front, and none are damaged or leaking. Since checking all these things I have found that the rears are still getting warmer than the fronts, though it hasn't gotten hot like the first time, even after a 30min highway drive. I'm wondering if the proportioning valve could be causing a problem by either sending too much braking to the rear, or causing the pressure not to release after the brakes are off. The truck has no lift, and is not ABS. I've searched, but haven't found any info on this particular problem. Any ideas?